Many marine biologists consider giant kelp, Macrocystis pyrifera, a foundation species that undergirds reef communities by creating habitat, sustaining ecosystem services and modifying environmental conditions to the benefit of other organisms. We tested this assertion using data from multiple long-term studies in the Santa Barbara Channel (SBC), California. We asked: (1) Do reefs forested with giant kelp support greater biodiversity than unforested reefs? (2) Does giant kelp play a central role in a reef’s ecological network? and (3) To what extent are kelp effects driven by the food or shelter kelp provides vs. correlated habitat associations? First, we tested the relationships between kelp biomass and the reef community with structural equation modeling using a 15-year time series of community-wide data at 39 plots. We then focused on a broad spatial scale (79 plots), to characterize the network of positive and negative associations between giant kelp and 49 other species, using Joint Species Distribution Models (JSDMs) to estimate: (1) significant associations between the giant kelp and other species that arise due to shared environmental responses, and (2) residual associations after accounting for shared environmental responses that could be attributed to biotic interactions.
We found that the effect of kelp on diversity was channeled through specific functional groups of reef organisms. Kelp biomass reduced understory algal communities through shading, which indirectly benefits sessile invertebrate communities and, in turn, mobile predators. JSDMs revealed that giant kelp was a peripheral rather than a connector species in the network of positive associations among kelp forest species. Most species associations at the large scale were explained by environmental variability, suggesting a dominant role of environmental sorting and shared environmental preferences. Yet, after accounting for environmental variability we found strong residual negative associations between giant kelp and many species, potentially due to strong species interactions. Taken together, these results suggest the effect of giant kelp on diversity might be more species specific and context dependent than thought. This contrasts with the expectation that giant kelp structures marine communities by providing unique food and shelter.